High sensitivity optical measurement techniques are required in the fabrication of electronic and optoelectronic devices. The manufacture of semiconductor devices typically begins with a large substrate wafer of semiconductor material, and a large number of semiconductor devices are formed in each wafer. Because the device manufacturing steps are time consuming and expensive, it is important to the manufacturing process of such semiconductor devices that the physical characteristics of semiconductor device structures only vary within a small process window. In order to attain the earliest possible feedback during production, it is necessary to non-destructively characterize the physical properties of device structures before the device is complete. Importantly, the active region of many electronic and optoelectronic devices are semiconductor quantum confined structures. For example, the gain media in communications lasers typically comprise multiple quantum well structures. Unfortunately, widely available optical spectrometry techniques such as ellipsometry or linear reflectance do not have the sensitivity required to observe signatures of quantum confined structures. The signatures of quantum confinement occur in the vicinity of strong interband transition features such as the band gap, and typically have small amplitudes, in some cases as small as one part in 106. Additional quantum confined optical signatures, arising from internal or interfacial electric fields also occur nearby to strong interband transition features. These signatures necessarily occur in the optical wavelength range, since semiconductor interband transitions occur in the ˜1-5 eV range. Thus, high sensitivity optical techniques are needed in the extraction of semiconductor quantum confined signatures.
The requirement for high sensitivity may be met by a proven class of optical techniques known as modulation spectroscopy techniques. Modulation spectroscopy techniques such as “electro-reflectance” and “photo-reflectance” have exhibited sensitivity to differential changes in reflectivity as small as 10−7. Of the modulation spectroscopy techniques, photo-reflectance is best suited for use in the fabrication of electronic and optoelectronic devices, as it is nondestructive and only requires the sample have a reflecting surface (Aspnes, 1980). The conventional photo-reflectance configuration employs a diode laser pump beam to induce small periodic changes in electron-hole populations. Amplitude modulation of the pump beam is conventionally accomplished with an optical chopper, or by fixturing a polarizer at the output of a phase modulator. A second optical beam, coincident with the modulated pump beam is then used to monitor small sample reflectivity changes using phase locked detection. Thus, the conventional photo-reflectance configuration is a realization of electro-modulation, wherein the electric field is induced by the space charge separation field of the electrons and holes (Pollack, 1996). This dual pump-probe beam approach increases the system complexity.
Another problem with conventional photo-reflectometers is they lack a tunable pump beam with wavelengths nearby to at least one strong interband transition feature, or “critical point,” in the band structure of the quantum confined structure. Importantly, quantum confinement shifts and sharpens spectral features near critical points in the optical absorption (Miller, 1982). Thus, by employing a tunable laser pump source with wavelength scanned across the critical point feature of interest in the band structure of the quantum confined structure, photo-reflectance information as a function of pump wavelength beam may be used to characterize the signatures of quantum confined structures.
Another problem with conventional photo-reflectometers is that they do not employ a polarization modulation technique to induce changes in the optical response of the quantum confined structure. By using polarization state modulation of the pump beam, and introducing the laser beam at a non-zero angle with respect to the surface normal, a component of polarization perpendicular to the sample surface is realized. Importantly, this polarization cannot be achieved by light incident normally onto the sample, as employed in conventional linear reflectance-difference spectroscopy (Aspnes, 1995). Generally, the optical response of a semiconductor quantum confined structure is anisotropic with respect to polarization vector. For example, when the quantum confined structure is a quantum well oriented parallel to the sample surface, light polarized in the plane of the quantum wells will couple to both “heavy-hole” and “light-hole” excitons, whereas light polarized normal to the plane of the well will couple preferentially to the “light-hole” excitons (Weiner, 1985). Thus, by introducing the polarization modulated laser beam at a non-zero angle with respect to the surface normal, anisotropy may be induced in the optical response of the quantum well. This feature of the polarization modulation technique is also useful for the characterization of strained layers, as strained layers are always quantum wells.
Thus, while conventional photo-reflectometers and optical spectrometers may be suitable for the particular purpose to which they address, they are not well suited for the characterization of the optical response of semiconductor quantum confined structures. The ability to record spectroscopic information using a single beam, which serves as both the pump and probe, greatly simplifies system operation. Moreover, although conventional photo-reflectometers may use a laser pump beam with sufficient power and focusing to create electric field modulation inside the sample, this pump is not wavelength tunable so as to characterize the signatures of quantum confined structures. Also, the laser pump is not polarization modulated so as to induce anisotropic optical responses associated with semiconductor quantum confined structures. Additionally, although conventional reflectance difference spectroscopy has been accomplished using polarization modulation, it has been implemented at normal incidence where no change in coupling to quantum confined states exists. Also, conventional reflectance difference spectroscopy uses lower laser powers.
In these respects, the polarization modulation photo-reflectometer according to the present disclosure substantially departs from the conventional concepts and designs of the prior art, and in so doing, provides an apparatus primarily developed for the characterization of optical signatures of quantum electronic confinement, including resolution of excitonic states at the band edge or other direct or indirect critical points in the band structure. This allows for characterization of quantum confinement, for characterization of strain in semiconductor films, and for characterization of electric fields at semiconductor interfaces.